Re: Is this a compiler error? "recursive template expansion"

[Nathan S. via Digitalmars-d-learn]

On Tuesday, 8 December 2020 at 22:01:52 UTC, Basile B. wrote:
On Tuesday, 8 December 2020 at 20:11:40 UTC, Nathan S. wrote:
The following code fails to compile. Is this a compiler error 
or if not what is wrong with the code?

What is wrong is that partial specialization is not correct.
The correct partial specialization is:

---
struct Template2(T)
{
    enum tString = T.stringof;
    static if (is(T == class))
        enum tLinkage = __traits(getLinkage, T);
}

struct Template1(Param1, Param2 = Template2!Param1) {}

alias AliasTemplate1S(SecondParam) = Template1!(S,SecondParam);
//                    ^here

class S
{
    Template1!int x;
}
---

Now that being said, the compiler could complain about the 
incorrect partial spec instead of falling in the nonsensical 
error message.

There is a gap because the second template param looks optional 
so you dont put it the partial specialization but it is still 
required.

Anyway. This case is either a "bad diagnostic" or an 
"enhancement" request.
Or should be specified.

Thanks a lot! In my case what I was intending was:

alias AliasTemplate1S = Template1!(S, Template2!S)

which as you suggest works fine. It's a bit odd that the 
non-optional second parameter becomes optional again if 
declaration order is shuffled, but my motivation to look into 
this has temporarily abated since it's no longer stopping me from 
doing something else.

For the program where I ran into this problem the most convenient 
fix turns out to be to get rid of the default template parameter 
and instead use a pattern like this:

---
struct Template1(Param1, Param2) {}
alias Template1(Param1) = Template1!(Param1, Template2!Param1);
---